2. Prior Art Many product packages are currently marked with batch and/or date codes, either as a legal requirement or as good manufacturing practice to allow the recall of faulty or contaminated products by batches, or to allow later tracing simply for quality control purposes. In some cases, the marking is printed on the product package by some appropriate printing technique, or burned into or onto the package by way of a laser marking system. Even stamping has been used, though this is now relatively rare because of its applicability to only certain types of packages and packaging equipment and the availability of simpler methods of marking.

One common type of product marking currently being used is a relatively conventional ink jet dot matrix printer-type marking. When working properly, such marking is good, though it has the disadvantages of requiring a drying or setting time before the same will not smear. It also has the disadvantage of its operation not being readily verifiable, and is subject to incomprehensible marking because of print head clogging. In the case of lasers, various types of product packages may be readily marked as desired, provided that the type of laser (wavelength and power) is appropriately selected for the surface to be marked.

By way of example, a C02 laser of approximately 10 watts of power and with an output wavelength of 10.6 microns will readily write directly onto a cardboard box such as is commonly used for packaging cereal and other food products, providing a high quality marking without penetrating the package. A C02 laser will also quickly burn through a thin layer of paint such as is commonly used for decorative purposes on soft drink cans to provide clear and durable markings thereon. In the case of writing directly to a metal surface such as directly onto the aluminum itself as used for soft drink and other products, a YAG laser is more appropriate.

Laser marking of product packages has the advantage of being fast, of providing a clean and accurate marking requiring no drying or set time, and of not easily being obliterated short of substantial damage to the product package. Heretofore however, laser marking also has had the disadvantage of not being readily verifiable short of visual observation of the marking, usually on randomly selected packages. Optical character recognition techniques may be used for verification purposes on ink jet or laser systems, but they are very costly, sometimes costing more than the coders. In that regard, while lasers and laser deflection systems are quite reliable and of course are not subject to drying up, clogging or the like, laser intensity does tend to drift with time, usually toward less beam intensity, and lasers have a limited useful duration between gas replenishment and the like. Further, while laser deflection systems are also quite reliable and will also generally operate without service somewhat longer than a laser, deflection systems are also not perfect, and could unexpectedly be subject to deteriorating performance or catastrophic failure. Accordingly, while laser marking systems offer certain distinct advantages, such as being able to cleanly and accurately mark product packages on the fly without either touching or being in very close proximity to the packages, prior art laser marking systems have shared the shortcoming of other prior art marking systems of not having a reliable and economical way of verifying continued proper operation of the marking system.

BRIEF SUMMARY OF THE INVENTION A laser marking system with beam deflection verification and automatic beam intensity control for marking products and product packaging with information such as batch numbers and date codes is disclosed. The laser coding system includes a computer controlled laser, together with a computer controlled laser beam deflection system, to directly write on product packages or products by burning the marking thereinto or through a coating thereon. The laser beam deflection system includes deflection system position sensors to provide outputs indicative of the then present position of the deflectors. These deflection system sensor outputs are correlated with the deflection system command signals to detect any abnormal deviations between the commanded positions and the actual positions indicative of a deflection system failure. A sensor is also provided for sensing the laser intensity to verify proper laser operation and to allow automatic laser power control measurement of the laser beam intensity by deflection of the beam to an out-of-range position for product marking, further verifying deflection system operation, as well as verifying laser operation and providing a signal for laser power control. Various features, such as the ability to verify operation while writing on moving product packages, and alternate embodiments, are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a representative embodiment of the present invention.

Figure 2 is a block diagram of an exemplary embodiment of the system of Figure 1.

Figure 3 is a block diagram of an alternate exemplary embodiment of the system of Figure 1.

Figure 4 illustrates one embodiment of a deflection system of the present invention.

Figure 5 illustrates an exemplary embodiment of the deflection system of Figure 4.

Figure 6 illustrates a second embodiment of a deflection system of the present invention.

Figure 7 illustrates a third embodiment of a deflection system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION First referring to Figure 1, a perspective view of a representative embodiment of the present invention may be seen. This embodiment happens to illustrate a marking system for marking directly on product packages 20, such as individual cereal boxes and the like, as the same pass the marking system on a conveyor 22. The marking system itself generally includes some form of support structure 24 which may or may not be part of the assembly for conveyor 22 and support legs 26. The structure 24 supports, in this case, a C02 laser 28, a laser control board 30 and laser power supply and computer 32. The structure 24 also supports an optical beam deflection system 34 on a vertical slide system, with the assembly 34 being adjustable in the vertical direction by such means as lead screw 36, controllably positioned by positioner 38. The positioner 38 may be a manually adjustable positioner, or an electrically controllable positioner such as a gear motor with feedback device, or stepper motor, so as to be automatically repositionable to a prior setting for lines which occasionally undergo a change-over to product packages of different sizes.

The optical beam deflection system 34 in this embodiment includes a right angle mirror reflector 44 and a two axis mirror deflection system 40, as well as one or more lenses 42 for focusing the laser beam to a small area spot on a product package in accordance with the positions of the beam deflector and the product package itself relative thereto. In the preferred embodiment, the focusing of the laser beam is done at one or more points after mirror 44, with the lens or lens system focusing the beam below the optical beam deflection system. Having the lens or lens system after mirror reflector 44, the collimated laser beam provided by laser 28 will remain collimated until after being bent 90 degrees by the reflector. Thus, the vertical position of the optical beam deflection system including mirror 44 relative to the laser 28 itself will not matter, so that the laser may be focused onto product packages of different sizes by simply raising or lowering the optical beam deflection system without requiring movement of the laser itself. In other embodiments, of course, the laser may be moved if desired, though in general a simpler assembly normally will result if this is avoided.

Now referring to Figure 2, a block diagram of the system of Figure 1 may be seen. The system is controlled by a computer 46, which for input/output and programming convenience may be a personal computer with keyboard, floppy disk drive and a temporary or permanent display for displaying such things as status of various parts of the system, the marking currently being commanded and the actual marking being written to the product packages passing thereby.

The computer 46 controls a galvanometer servo card 82. The digital signals from the computer are converted to analog signals in the servo card since the galvanometers are analog devices. The servo card provides analog signals to control the x-axis galvanometer 48 and the y-axis galvanometer 50. A front surface mirror is firmly attached to each galvanometer shaft. The galvanometers controllably rotate the x-axis mirror 52 and y-axis mirror 54 about orthogonal axes to provide x-axis and y-axis deflection of the beam from laser 28 to write on the package 20 passing thereunder.

The galvanometers include an x-axis shaft positioning sensor 56 and a y-axis shaft position sensor 58, which in the preferred embodiment are capacitive position sensors. Other types of sensors may also be used if desired, such as by way of example, photoelectric devices. The sensors provide analog position feedback signals of the orthogonal x and y axes to the servo card 82. The servo card utilizes the position sensor signals to form a closed loop servo system for each axis. Additionally, the servo card electronics differentiates the position signals from each position sensor and uses this rotational velocity feedback to critically damp the galvo/mirror systems.

This allows the x and y servo systems to accurately position the x and y mirrors, with no overshoot, based on the output signals from the computer. It may be noted that the galvanometer servo card 82 could operate in the digital mode with digital galvanometer position feedback instead of converting the computer digital signals to the analog format, though the preferred embodiment uses servo card operating in the analog mode.

The analog x-axis shaft position and y-axis shaft position signals from sensors 56 and 58 are also provided to x-axis shaft correlator 60 and y-axis shaft correlator 62, respectively, which correlate (or verify) the measured shaft positions with the commanded shaft positions to be sure that the galvanometer mirrors are tracking the command signals to the degree of accuracy expected. It should also be noted that mirrors are firmly attached to the galvanometer shafts. These x- axis and y-axis shaft correlators provide verification warning outputs 64 and 66, which typically will be used to provide warning lights, a warning buzzer and/or even temporarily shut down the line, and perhaps provide a better indication of the difficulty on the display.

As shown in Figure 2, the correlators 60 and 62 receive the respective digital galvanometer command signals from computer 46 and the respective analog galvanometer shaft position signals from the shaft position sensors 56 and 58. The analog galvanometer shaft position signals are converted to a digital format since it is easier to correlate digital formatted signals. If a digital based servo system was used then this conversion would not be necessary.

It should be noted that the words"correlator} or "correlation"are used herein and in the claims in the most general sense, as opposed to a purely mathematical sense, though of course true analog or digital correlators in the mathematical sense could be used if desired. However, much simpler correlation techniques are readily applicable to the present invention. In that regard, the object, of course, is to assure that the galvanometer mirrors actually move in accordance with the commands provided thereto, or more specifically, so that the difference at any time between the galvanometer shaft position and the commanded shaft position is less than a predetermined amount. Since the galvanometers are critically damped, their output will lag the input. This time response lag can be compensated for in the logic circuits of the correlators. The variation or error difference between the input command signals to the galvanometers and the output signals from the shaft position sensors can be set in the correlator logic to any desired figure, based on a user's requirements.

For further verification and troubleshooting, the x-axis shaft position and y-axis shaft position of the galvanometers could be monitored and processed so that not only the commanded shaft positions could be displayed on a display, but the shaft position sensor signals could also be displayed, giving better insight into the actual system problems in the event one or both correlators indicated a servo malfunction. Also, if desired, the correlators could be made functional only when the laser is operating, as deflection system operation when the laser is off may intentionally command more rapid motion, causing justifiably greater deviation between commanded position and actual position between writing strokes of the laser.

In Figure 2, the correlators are shown as separate blocks, and of course may be separate circuits or devices as desired. However, the galvanometer servo card 42 has available thereon the analog form of the commanded x-axis and y-axis galvanometer positions, as well as the analog outputs of the x-axis shaft position and y-axis shaft position sensors 56 and 58.

Accordingly, the analog signals for each axis may be compared by a differential amplifier for each axis, with limit gates on the output of the differential amplifiers triggering whenever the predetermined limits are exceeded. The time lag between the signals must be considered. In general, once the limits are exceeded, the warning should be set, though the computer 46 could be used to monitor an automatically resetting warning signal to determine whether the warning was set by a consistent failure, an erratic failure or simply a one- time malfunction. As a possible alternate, because the difference between the commanded positions and the actual positions of the deflectors is always quite small for a proper functioning deflection system, the x and y actual positions could be combined, such as by adding, and compared to a similar combination of the x and y commanded positions to enable the use of a single correlator to provide a single deflection system failure indication. If desired, the correlation could verify the laser marking on a character-by-character basis for absolute conditions.

The laser 28 is turned on and off by the computer 46 through laser control 68. To be sure that the laser beam is working properly however, it is desirable to sense the laser emission. It is further desirable to provide a measure of the laser intensity so that the laser power may be adjusted accordingly through a laser power control 70 controlled by the computer 46. This may be done in various ways. By way of example, a sensor may be placed at the back end of some types of lasers, as a small but detectable fraction of the emission of the laser will escape through the mirror at the back end of the laser. Alternatively, the right angle deflection mirror 44 (see Figure 1) may be imperfect, with a sensor placed thereunder to detect the small amount of the emission passing through the mirror to the sensor. While these, and other techniques, would be functional to assure proper operation of the laser, they would still leave the possibility of a galvanometer mirror failure, that is, a shift or total separation of a galvanometer mirror from the supporting shaft of the galvanometer. Such a failure may not be detected by merely comparing galvanometer shaft angle to commanded galvanometer position, unless the mirror somehow interfered with the shaft rotation. Thus, in the preferred embodiment, an emission sensor 72 is placed in the two-axis beam deflection system 40 (see Figure 1) just to the side of the lens or lens system 42, so as to not interfere with the laser beam when actually marking product packages, but to be still within the deflection range of the two axis deflection system so as to allow the deflection of the laser beam to the emission sensor 72. This allows the intensity of the laser emission to be measured by the computer 46.

Preferably the emission sensor 72 will be placed at a 45 degree angle with respect to the x and y axes, so as to require adequate deflection of both deflection systems in order to illuminate the sensor with the laser beam. If desired, of course, two diametrically opposed sensors could be used so that both galvanometers could be exercised to their full range for both laser and galvanometer range testing. Also, if desired, the galvanometer position sensors may be placed on the mirrors themselves, rather than on the galvanometer shafts, so that a mirror separation will be immediately detected by the position sensor. In that regard, the position sensor for the galvanometer feedback loop may be conveniently used for the correlators as shown in Figure 2, though different position sensors could be used for the galvanometer feedback loop and for the correlators. For instance, galvanometer shaft position sensors could be used for the servo loops, and mirror position sensors 88 and 90 used for the correlators, as shown in Figure 3. This has the advantages of not only detecting mirror separation or other mirror irregularities, but also of detecting irregularities in either position sensor that may not be detectable if the same position signal is used for both purposes.

In any event, laser operation and intensity may be checked by the computer between marking adjacent product packages if the conveyor speed is not too fast.

Alternatively, because lasers are relatively reliable, though tend to drift and/or decrease in intensity with time, the laser beam might be deflected to the emission sensor 72 on a less frequent basis, such as once per day. In general, in response to a failure of the laser or a drift downward in the laser intensity to a value approaching the limits of the computer to automatically control the laser intensity, some appropriate warning system 74 controlled by the computer is provided. Of course, on an actual laser or deflection system malfunction, the line might be automatically shut down, though on a mere indication of the laser approaching a need for service, operation could continue with the computer providing status information for timely servicing or replacement of the laser.

Since in the preferred embodiment the deflection of the laser beam to the emission sensor or sensors 72 is an extraordinary deflection, it might be desirable to allow greater deviation between the commanded beam position and the actual galvanometer position. This, of course, may be readily controlled by computer 46 if desired. Further, it should be noted that the laser beam will not be well focused on the emission sensor 72, an actual advantage in avoiding damage to the sensor.

In the present invention, the marking of product packages under the laser beam occurs as the product packages are moving at a relatively constant speed for each package, though that speed may change from time to time over the passage of a number of packages, including but not limited occasions of line startup and shutdown.

A conveyor encoder 78 is used to compensate for changes in package velocity. Deflection of a laser beam to write on either a stationary or a moving object is a well known technology, and accordingly will not be described in detail herein. In the present invention, for writing on an object moving in one direction, the writing should preferably proceed on the package in the other direction, as this minimizes the required beam deflection along the axis of motion, and generally allows the maximum package speed for a particular amount of writing.

Figure 4 illustrates one embodiment of a deflection system of the present invention. Referring to Figure 4, the deflection system is a two axis mirror deflection system and includes a x-axis mirror 52, a x-axis galvanometer 48 that controls the x-axis mirror 52, a y- axis mirror 54, a y-axis galvanometer 50 that controls the y-axis mirror 54, and a lens system 42 that positions and focuses a laser beam 84 onto a moving product 20 for marking the product surface 86. Although Figure 4 shows the laser beam 84 being deflected by the x-axis mirror 52 first, it is contemplated that the laser beam can alternatively be deflected by the y-axis mirror 54 first. The deflection system further includes a laser beam emission sensor 72 placed at a test point to the side of the lens system 42. The laser beam emission sensor 72 is placed within the deflection range of the deflection system such that the laser beam 84 can be deflected onto the test point (preferably using substantial deflections of both the x-axis and y-axis mirrors) in order to verify that the electronics (e. g., the servo card 82 of Figure 2) and/or the deflection system is working properly and to measure the intensity of the laser beam.

It must be noted that more than one emission sensor may be used such as x-and y-axis mirror position (or emission) sensors 88 and 90 of Figure 5. In such a combination of sensors, the galvanometer zero positions and scale factor may be determined and compensated for under software control without accurate mechanical alignment or scale factor trimming. Moreover, in the event of galvanometer failure, the x-axis and y-axis galvanometer emission sensors 88 and 90 can be used to determine which (or both) galvanometers are malfunctioning.

If the laser beam emission sensor 72 does not receive the appropriate verification, i. e., when the laser beam is not detected or the laser beam's intensity is not within a predetermined range, the laser beam emission sensor 72 causes an appropriate warning system 74 to be provided. If the laser beam's intensity appears to deviate from a predetermined control point, the laser beam power control 70 can be automatically adjusted until the laser beam's intensity is within the predetermined range. Thus, an appropriate warning will typically be provided if the laser beam's intensity cannot be adjusted to within the predetermined range.

Figure 6 illustrates a second embodiment of a deflection system of the present invention. Referring to Figure 6, the deflection system includes a mirror 98 for deflecting the laser beam 84 in one axis, a galvanometer 100 that controls the mirror 98, and a rotating polygon 92 having a plurality of mirrored faces 94 for deflecting the laser beam 84 in the other axis.

The laser beam 84 is switched in synchronism with the rotating mirrors of the polygon 92. The rotating polygon 92 may be used as a x-axis or y-axis deflector.

Figure 7 illustrates a third embodiment of a deflection system of the present invention. Referring to Figure 7, the deflection system includes a mirror 98 for deflecting the laser beam 84 in one axis, a galvanometer 100 that controls the mirror 98, and an acousto-optic deviator 96 (e. g., Germanium Crystal) that deflects the laser beam 84 in the other axis. The acousto-optic deviator 96 may be used as a x-axis or y- axis deflector. Preferably, the acousto-optic deviator 96 deflects the laser beam in an axis that defines the height of characters that are marked on products (i. e., the axis that requires a smaller range of deflection).

The net result of the invention herein described is that substantially all likely failures in laser marking systems may be detected, including a failure of the laser itself, a failure in the laser power control, a failure in either axis of the beam deflector, including not only failures of the galvanometers, but also even separation of the galvanometer mirrors actually deflecting the beam. The warning and/or action taken upon detection of an abnormal operating condition, of course, will depend upon the nature of the abnormality, and in some cases, the nature of the products being marked. If desired, of course, other failure sensing devices or circuits may be included with the present invention. By way of example, it is possible that computer 46 itself might fail, providing a constant output on the bus and thus, a non-changing input to the galvanometer servo card 42 and to the correlator. The non-changing position of the galvanometer, under such conditions, would of course not trigger the correlator.

However, other circuits may readily detect such a condition, such as by way of example, a simple detection of a minimum average AC level on either the analog galvanometer command signals and/or the analog galvanometer shaft position outputs, which circuits preferably would be separate from computer 46 and separately powered.

Thus while the present invention has been disclosed and described with respect to certain preferred embodiments thereof, it will be understood by those skilled in the art that the present invention may be varied without departing from the spirit and scope thereof.